The present invention pertains to a method and an arrangement for providing detection of deteriorated lamp filaments in a lamp circuit fed by constant current, especially for incandescent lamps comprised in airport lighting systems.
At airports, lighting systems are used for directing airplanes during landing and taxiing. These lighting systems have a large number of lamps and it is important that failed lamps are replaced quickly, especially during times of low visibility. Otherwise, the consequences of a plane missing a taxiway or a stop signal can be disastrous. Since visual lamp inspection increases the risk for an accident and induce costs, automatic lamp monitoring systems have been developed.
Lamps in these lighting systems are frequently connected into a so-called series circuit using an isolation transformer for each lamp. Such lamps are connected in series via a power cable and fed by a constant current power supply from a constant current regulator (CCR). During a lamps burning time its brightness deteriorates. This is partly due to the fact that material evaporates from the filament, which usually has a helical shape, and sublimates onto the glass bulb where it absorbs part of the emitted light. The probability for this to happen is, however, rather low as the type of lamp used is designed such that the evaporated material sublimates onto the filament again.
A more probable reason for lamp degradation is that the evaporated material sublimates in such a way that shorting bridges are formed between adjacent filament coil turns. If a part of the filament in a lamp fed with constant current is shorted, the nominal wattage of the lamp is reduced as it is proportional to the resistance of the filament. Hence, the lamp intensity will be reduced. Eventually the lamp fails due to a complete shortage or, more often, a breakage in the filament. This is opposed to the case when a lamp is fed with constant voltage. Then its nominal wattage is inversely proportional to the filament resistance (Intensity≈wattage2), and shortage of the filament will cause it to burn off more or less immediately due to an excessive power dissipation.
In the case a lamp fails due to a breakage in the filament, a high voltage arises on the primary and on the secondary side of the isolating transformer connected to the lamp, as the impedance on the secondary side of the transformer then becomes infinite. High voltage appears at the beginning of each cycle of an AC current fed through the transformer.
Isolating transformers used, however, can be designed such that a high voltage only appears for a short time whereafter the transformer core is saturated. When the core is saturated the voltage across the transformer drops to a low value as the impedance of the transformer thus is low.
In case a lamp fails due to a shortage of the filament, the voltage across the transformer is only slightly reduced. This is true when there is a considerable voltage drop in a long cable between the transformer and a lamp.
Known lamp monitoring systems, which are for sale, detect lamp failures due to filament breakage. A common type monitors the current and the voltage supplied by the constant current, and the voltage supplied by the constant current power supply to the series circuit, thereby detecting impedance changes in the circuit caused by failed lamps.
A further type of monitoring system includes a monitoring unit located at each lamp, where the monitoring unit detects a voltage increase that occurs at each half period of the current before the isolating transformer core saturates, or the monitoring system simply detects a xe2x80x9cno currentxe2x80x9d condition in a open circuit.
None of the systems detect partly or completely shorted filaments.
As lamps deteriorate in a fairly predictable way before they fail there are also systems that predict lamp failure based on a lamps burn time. A record is held for each series circuit regarding the accumulated time the circuit has been switched on. In an advanced version, a separate accumulated time record is held for a maximum current value, or for each current value at which the circuit is energised. Based on these records the lamps are replaced after a certain, empirically established, burn time.
A complicating factor is that lighting systems are becoming common which allow for selective switching, i.e., not all lights in a series circuit are turned on and off together. Only those lights that are necessary to guide an aircraft at a particular moment are turned on and off at the same time, which makes it rater complicated to predict the remaining lifetime for each lamp based on its burn time combined with the intensity used.
The present invention provides detection of deteriorated lamp filaments in accordance with attached independent method and arrangement claims. Embodiments of the invention are defined through attached dependent claims.
The present invention attains to provide a detection method for a monitoring system, which makes it possible to detect a partially or completely shorted lamp in a lamp circuit fed by constant currents.
In doing so, a method is set forth, providing detection of deteriorated lamp filaments in a lamp circuit fed by constant current. When a change in the constant current fed through the lamp circuit is initiated, the resistance, once in conjunction with the current, change and once a predetermined time period later is determined or measured, whereby the difference between the resistance determinations or measurements constitutes the deterioration of a lamp filament in comparison with a threshold value for said difference, thereby avoiding a record keeping of previous resistance measurements.
One embodiment of the invention comprises that a lamp should be replaced when said resistance difference is equal to or smaller then the threshold value.
A further embodiment comprises that a resistance is determined by measuring the voltage across a lamp circuit and across a resistor placed in series with the lamp circuit, whereby the quotient between said determined values multiplied with the value of said resistor is equal to the resistance of the lamp circuit.
A still further embodiment comprises that said threshold value is different for different current values.
Another embodiment comprises that said threshold value is different for different nominal wattage of lamps.
Yet in one other embodiment said threshold values are empirically established for each current value.
Also, according to the present invention, a monitoring arrangement is set forth providing detection of deteriorated lamp filaments in a lamp circuit fed by constant current. It comprises:
lamp monitoring means connected to said lamp circuit, which detects that a change in said constant current is initiated to flow through said lamp circuit;
resistance determining or measuring means, which determine or measure a resistance value across a lamp circuit once in conjunction with said current change and once a predetermined time period later;
difference determining or measuring means measuring a difference value between said determined or measured resistance values;
evaluation means for evaluating said resistance difference value compared with threshold value for said difference; and
whereby the difference between the resistance measurements constitutes the deterioration of a lamp filament compared with said threshold value, thereby avoiding a record keeping of previous resistance measurements.
Further the arrangement is able to set forth steps of the above method as described in the method claims.